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Abstract

Preservation of organic matter in estuarine and coastal areas is an important process in the global carbon
cycle. This paper presents bulk d13C and C/N of organic matter from source to sink in the Pearl River
catchment, delta and estuary, and discusses the applicability of d13C and C/N as indicators for sources of
organic matter in deltaic and estuarine sediments. In addition to the 91 surface sediment samples, other
materials collected in this study cover the main sources of organic material to estuarine sediment. These
are: terrestrial organic matter (TOM), including plants and soil samples from the catchment; estuarine
and marine suspended particulate organic carbon (POC) from both summer and winter. Results show
that the average d13C of estuarine surface sediment increases from 25.0 1.3& in the freshwater
environment to 21.0 0.2& in the marine environment, with C/N decreasing from 15.2 3.3 to
6.8 0.2. In the source areas, C3 plants have lower d13C than C4 plants (29.0 1.8& and 13.1 0.5&
respectively). d13C increases from 28.3 0.8& in the forest soil to around 24.1& in both riverbank
soil and mangrove soil due to increasing proportion of C4 grasses. The d13CPOC increases from
27.6 0.8& in the freshwater areas to 22.4 0.5& in the marine-brackish-water areas in winter, and
ranges between 24.0& in freshwater areas and 25.4& in brackish-water areas in summer. Comparison
of the d13C and C/N between the sources and sink indicates a weakening TOM and freshwater POC
input in the surface sedimentary organic matter seawards, and a strengthening contribution from the
marine organic matter. Thus we suggest that bulk organic d13C and C/N analysis can be used to indicate
sources of sedimentary organic matter in estuarine environments. Organic carbon in surface sediments
derived from anthropogenic sources such as human waste and organic pollutants from industrial and
agricultural activities accounts for less than 10% of the total organic carbon (TOC). Although results also
indicate elevated d13C of sedimentary organic matter due to some agricultural products such as sugarcane,
C3 plants are still the dominant vegetation type in this area, and the bulk organic d13C and C/N is
still an effective indicator for sources of organic matter in estuarine sediments.